test(background-agent): add unit tests for extracted modules

Add 104 new tests across 4 test files:
- error-classifier.test.ts (80 tests): isRecord, isAbortedSessionError, getErrorText, extractErrorName, extractErrorMessage, getSessionErrorMessage
- fallback-retry-handler.test.ts (19 tests): retry logic, fallback chain, concurrency release, session abort, queue management
- process-cleanup.test.ts (7 tests): signal registration, multi-manager shutdown, cleanup on unregister
- compaction-aware-message-resolver.test.ts (13 tests): compaction agent detection, message resolution with temp dirs (pre-existing, verified)

Total background-agent tests: 161 -> 265 (104 new, 0 regressions)

src/features/background-agent/process-cleanup.test.ts

@@ -0,0 +1,156 @@
+import { describe, test, expect, beforeEach, afterEach, mock } from "bun:test"
+import {
+  registerManagerForCleanup,
+  unregisterManagerForCleanup,
+  _resetForTesting,
+} from "./process-cleanup"
+
+describe("process-cleanup", () => {
+  const registeredManagers: Array<{ shutdown: () => void }> = []
+  const mockShutdown = mock(() => {})
+
+  const processOnCalls: Array<[string, Function]> = []
+  const processOffCalls: Array<[string, Function]> = []
+  const originalProcessOn = process.on.bind(process)
+  const originalProcessOff = process.off.bind(process)
+
+  beforeEach(() => {
+    mockShutdown.mockClear()
+    processOnCalls.length = 0
+    processOffCalls.length = 0
+    registeredManagers.length = 0
+
+    process.on = originalProcessOn as any
+    process.off = originalProcessOff as any
+    _resetForTesting()
+
+    process.on = ((event: string, listener: Function) => {
+      processOnCalls.push([event, listener])
+      return process
+    }) as any
+
+    process.off = ((event: string, listener: Function) => {
+      processOffCalls.push([event, listener])
+      return process
+    }) as any
+  })
+
+  afterEach(() => {
+    process.on = originalProcessOn as any
+    process.off = originalProcessOff as any
+
+    for (const manager of [...registeredManagers]) {
+      unregisterManagerForCleanup(manager)
+    }
+  })
+
+  describe("registerManagerForCleanup", () => {
+    test("registers signal handlers on first manager", () => {
+      const manager = { shutdown: mockShutdown }
+      registeredManagers.push(manager)
+
+      registerManagerForCleanup(manager)
+
+      const signals = processOnCalls.map(([signal]) => signal)
+      expect(signals).toContain("SIGINT")
+      expect(signals).toContain("SIGTERM")
+      expect(signals).toContain("beforeExit")
+      expect(signals).toContain("exit")
+    })
+
+    test("signal listener calls shutdown on registered manager", () => {
+      const manager = { shutdown: mockShutdown }
+      registeredManagers.push(manager)
+
+      registerManagerForCleanup(manager)
+
+      const [, listener] = processOnCalls[0]
+      listener()
+
+      expect(mockShutdown).toHaveBeenCalled()
+    })
+
+    test("multiple managers all get shutdown when signal fires", () => {
+      const shutdown1 = mock(() => {})
+      const shutdown2 = mock(() => {})
+      const shutdown3 = mock(() => {})
+      const manager1 = { shutdown: shutdown1 }
+      const manager2 = { shutdown: shutdown2 }
+      const manager3 = { shutdown: shutdown3 }
+      registeredManagers.push(manager1, manager2, manager3)
+
+      registerManagerForCleanup(manager1)
+      registerManagerForCleanup(manager2)
+      registerManagerForCleanup(manager3)
+
+      const [, listener] = processOnCalls[0]
+      listener()
+
+      expect(shutdown1).toHaveBeenCalledTimes(1)
+      expect(shutdown2).toHaveBeenCalledTimes(1)
+      expect(shutdown3).toHaveBeenCalledTimes(1)
+    })
+
+    test("does not re-register signal handlers for subsequent managers", () => {
+      const manager1 = { shutdown: mockShutdown }
+      const manager2 = { shutdown: mockShutdown }
+      registeredManagers.push(manager1, manager2)
+
+      registerManagerForCleanup(manager1)
+      const callsAfterFirst = processOnCalls.length
+
+      registerManagerForCleanup(manager2)
+
+      expect(processOnCalls.length).toBe(callsAfterFirst)
+    })
+  })
+
+  describe("unregisterManagerForCleanup", () => {
+    test("removes signal handlers when last manager unregisters", () => {
+      const manager = { shutdown: mockShutdown }
+      registeredManagers.push(manager)
+
+      registerManagerForCleanup(manager)
+      unregisterManagerForCleanup(manager)
+      registeredManagers.length = 0
+
+      const offSignals = processOffCalls.map(([signal]) => signal)
+      expect(offSignals).toContain("SIGINT")
+      expect(offSignals).toContain("SIGTERM")
+      expect(offSignals).toContain("beforeExit")
+      expect(offSignals).toContain("exit")
+    })
+
+    test("keeps signal handlers when other managers remain", () => {
+      const manager1 = { shutdown: mockShutdown }
+      const manager2 = { shutdown: mockShutdown }
+      registeredManagers.push(manager1, manager2)
+
+      registerManagerForCleanup(manager1)
+      registerManagerForCleanup(manager2)
+
+      unregisterManagerForCleanup(manager2)
+
+      expect(processOffCalls.length).toBe(0)
+    })
+
+    test("remaining managers still get shutdown after partial unregister", () => {
+      const shutdown1 = mock(() => {})
+      const shutdown2 = mock(() => {})
+      const manager1 = { shutdown: shutdown1 }
+      const manager2 = { shutdown: shutdown2 }
+      registeredManagers.push(manager1, manager2)
+
+      registerManagerForCleanup(manager1)
+      registerManagerForCleanup(manager2)
+
+      const [, listener] = processOnCalls[0]
+      unregisterManagerForCleanup(manager2)
+
+      listener()
+
+      expect(shutdown1).toHaveBeenCalledTimes(1)
+      expect(shutdown2).not.toHaveBeenCalled()
+    })
+  })
+})